Abstract:

Many metamorphic and sedimentary soft rocks exhibit significant time de-pendent behavior such as creep and relaxation which affect the stability of rock struc-tures. In the design of a durable structure in soft grounds, the secondary stage of creep is of paramount interest to predict the stability over long periods of time. The design of support system to resist such a time dependent loads is also of prime importance. This formula contains two material constants; activation energy and stress power. In this study, it is tried to find the activation energy of a rock salt sample using multistage tests at elevated temperature. By performing such creep test on several samples under different stress levels and finding stress power, creep parameters with minimum amount of rock samples are achieved. These kinds of tests have been done by both compression and impression testsetups, and the results are compared in the present study.

1 INTRODUCTION

Many sedimentary and metamorphic rocks exhibit significant creep behavior over time. This will affect the stability of rock structures due to strain accumulation. In the roof and walls of tunnels and caverns this may ultimately causerock falls and collapse. This matter is of particular importance for rock caverns used for oil and gas storage and for deposition of hazardous waste products as well as the public underground spaces. Traditionally, the most common way of investigating creep behavior is to carry out a static load, or ‘creep’, test; that is, holding a constant differential stress on a cy-lindrical rock sample and measuring the resulting strain as a function of time. Figure 1 shows a schematic behavior of materials under constant load, involving three individ-ual stages of creep, labeled transient (primary or decelerating); steady-state (secondary or constant rate); and accelerating (tertiary or unstable).

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